May 16, 2020

Conquering Complacency in Construction: The Importance of Being Earnest

3 min
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In the United Kingdom, the global recession seems to be over and the construction industry has experienced 16 months of sustained growth. However, this...

In the United Kingdom, the global recession seems to be over and the construction industry has experienced 16 months of sustained growth. However, this means that it is now starting to experience problems usually associated with a boom; sub-contractor prices increasing, increasing delivery times for materials and a shortage of skilled labour.   

In the current economic climate, clients are eager to start construction on site and financiers are equally keen to lend them money.  What both clients and financiers seem to forget, however, is that it might be prudent to have a signed building contract recording what has been agreed upon before they are about to start on site.  When they do realise this oversight and contact me, I am usually advised that the contract is agreed.  Yet it is only when I start to ask about insurances, liquidated damages, commencement and completion dates, that it becomes apparent that there has been some misunderstanding, despite the rest of the project team seeming to be eternal optimists who are sure that the project will be alright.

For example, a theatre group recently contacted me wanting to check over a contract which had already been agreed.  They had a six week slot to carry out refurbishment works; the only problem was, I soon discovered, that their chosen contractor had priced the job working to a 12-week programme.  As a consequence, the stage set for their production of ‘The Importance of Being Earnest’ would have been on a building site!

This lack of foresight leads to the inevitable ‘letter of intent’ which is always a potential source of dispute, ample evidence of which can be found in the law reports from the Technology and Contraction Court. If a letter of intent is to be anything other than a mere statement of future intent without any contractual obligation then it needs to deal with the same issues as a building contract: what work is to be carried out under the letter and what price is to be paid for it? What valuation and payment procedures are to be adopted? 

If materials are to be ordered when do they become the property of the employer and when does risk pass to the employer? How are the obligations under the letter to be terminated or cease to be of effect?  What insurance is to be effected and what liabilities do the parties have to each other?  On what basis does the contractor occupy the site? What are the dispute resolution procedures? 

This may seem that lawyers are being earnest about the contractual paper work and therefore generally obstructive when everyone else wants to get on with the project, but at least one of my clients was good deal happier after they telephoned last Friday; his contractor had gone into administration and what could he do?  I was able to tell him that they did have a signed contract and collateral warranties and advised him of what could be done as a result. 

My assistant and I spent a good deal of time prior to this exercising step-in-rights in consultant and sub-contractor warranties, securing the site and issuing withholding notices so no further payments were due to the, now insolvent, contractor, but it was a good deal easier and the delay to the project was minimised compared to the problems we would have had if there had been no contract at all. 

And why had this unfortunate contractor become insolvent?  Apparently he had underestimated his material costs and sub-contractor rates when tendering for the project six months ago!  

Matthew Needham-Laing is Head of Construction & Engineering at Stevens & Bolton LLP

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Jun 17, 2021

Why engineers must always consider human-induced vibration

Dominic Ellis
3 min
Human-induced vibration can lead to a number of effects upon the structure and its users

Human induced vibration, or more accurately vibrations caused by human footfall, often conjures images of Millennium Bridge-style swaying or collapsing buildings.

But in reality, the ‘damage’ caused by human-induced vibrations is less likely to ruin a structure and more likely to cause discomfort in people. Though not as dramatic as a structural failure, any good engineer wants to make sure the people using their structures, be it bridges or buildings or anything in between, can do so safely and comfortably. This is why human-induced vibration must be considered within the design process.

Resonance v Impulse

There are two ways that human-induced vibrations affect structures: resonant, and impulse or transient response. Put simply, resonance occurs when Object A vibrates at the same natural frequency as Object B.

Object B resonates and begins to vibrate too. Think singing to break a wine glass! Although the person singing isn’t touching the glass, the vibrations of their voice are resonating with the glass’s natural frequency, causing this vibration to get stronger and stronger and eventually, break the glass. In the case of a structure, resonance occurs when the pedestrian’s feet land in time with the vibration.

On the other hand, impulse or transient vibration responses can be a problem on structures where its natural frequencies are too high for resonance to occur, such as where the structure is light or stiff. Here the discomfort is caused by the initial “bounce” of the structure caused by the footstep and is a concern on light or stiff structures.

Engineers must, of course, design to reduce the vibration effects caused by either impulse or resonance.

Potential impacts from human induced vibration

Human induced vibration can lead to a number of effects upon the structure and its users. These include:

  • Interfering with sensitive equipment Depending on the building’s purpose, what it houses can be affected by the vibrations of people using the building. Universities and laboratories, for example, may have sensitive equipment whose accuracy and performance could be damaged by vibrations. Even in ordinary offices the footfall vibration can wobble computer screens, upsetting the workers.
  • Swaying bridges One of the most famous examples of human-induced resonance impacting a structure occurred with the Millennium Bridge. As people walked across the bridge, the footsteps caused the bridge to sway, and everybody had to walk in time with the sway because it was difficult not to. Thankfully, this feedback can only occur with horizontal vibrations so building floors are safe from it, but footbridges need careful checking to prevent it.
  • Human discomfort According to research, vibrations in buildings and structures can cause depression and even motion sickness in inhabitants. Tall buildings sway in the wind and footsteps can be felt, even subconsciously by the occupants. It has been argued that modern efficient designs featuring thinner floor slabs and wider spacing in column design mean that these new builds are not as effective at dampening vibrations as older buildings are.
  • Jeopardising structural integrity The build-up of constant vibrations on a structure can, eventually, lead to structural integrity being compromised. A worse-case scenario would be the complete collapse of the structure and is the reason some bridges insist that marching troops break step before crossing. Crowds jumping in time to music or in response to a goal in a stadium are also dynamic loads that might damage an under-designed structure.

How to avoid it

As mentioned, modern designs that favour thinner slabs and wider column spacing are particularly susceptible to all forms of vibration, human-induced or otherwise, but short spans can also suffer due to their low mass. Using sophisticated structural engineering software is an effective method for engineers to test for and mitigate footfall and other vibrations at the design stage.

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